Length-Tension Relationship
Length-Tension Relationship
• Amount of tension and force of contraction depends on how
stretched or contracted muscle was before it’s stimulated
Optimum resting length
z
z
• If overly contracted at rest, a weak contraction results
Overly contracted
z
• if too stretched before stimulated, a weak contraction results
z
Overly stretched
z
– If Sarcomre is < 60% or >175% of original length develop no
tension
z
1.0
• optimum resting length produces greatest force when muscle
contracts
Tension
generated
0.5
– CNS adjusts our muscle tone – state of partial contraction
– maintains optimum length – i.e., always ready for action
0.0
1.0
2.0
3.0
Sarcomere length (µm) before stimulation
4.0
11-1
11-2
Behavior of Whole Muscles
Phases of twitch contraction
• Latent Period: time
required for excitation,
excite/coupling and tension
- internal tension, no
shortening
• twitch – a quick cycle of contraction and relaxation
• Contraction phase – sliding
filamants
- external tension
• subthreshold electrical stimulus causes = no
contraction
• Relaxation phase - SR
reabsorbs Ca+2, myosin
releases actin - - tension
declines
– muscle returns to
resting length
– twitch lasts from 7 to
100 msec
Latent
period
Time of
stimulation
Time
Figure 11.13
11-3
at threshold intensity and above - a twitch is produced
– twitches caused by increased voltage are no stronger than those at
threshold
– i.e. muscles follow all-or-none law (contracting to its maximum or not at
all)
Stimulus voltage
subthreshold stimulus – no contraction at all
•
11-4
Recruitment and Stimulus Intensity
Contraction Strength of Twitches
•
Relaxation
phase
Contraction
phase
Muscle tension
• threshold - minimum voltage necessary to generate an
action potential in the muscle fiber and produce a
contraction
Threshold
1
2
3
4
5
6
7
8
9
Stimuli to nerve
Figure 11.14
Proportion of nerve fibers excited
not exactly true
– twitches vary in strength depending upon:
• stimulus frequency - stimuli arriving closer together produce
stronger twitches
• concentration of Ca+2 in sarcoplasm can vary the frequency
• how stretched muscle was before it was stimulated
• temperature : warmed-up muscle contracts more strongly –
enzymes work more quickly
• pH: if low in sarcoplasm weakens contraction - fatigue
• hydration of muscle affects overlap of actin & myosin
Maximum contraction
Tension
•
1
2
3
4
5
6
7
8
9
Responses of muscle
• stimulating nerve with higher and higher voltages produces stronger
contractions
– higher voltages excite more nerve fibers in the motor nerve which stimulates
more motor units to contract
11-5
• recruitment or multiple motor units (MMU) summation – bringing
more motor units into play
11-6
1
Twitch Strength & Stimulus Frequency
Treppe: stimulus right after relaxation following
tension is greater than previous
Twitch
Muscle twitches
Muscle develops
tension but does
not shorten
Muscle shortens,
tension remains
constant
Muscle lengthens
while maintaining
tension
Movement
Movement
No movement
Stimuli
(a)
(b)
Incomplete tetanus: stimuli before muscle relaxes =
summation of twitches – rapid cycles of contr. & relax.
Tension rises!
Complete tetanus: freq so great there is no relaxation
(a) Isometric contraction
(b) Isotonic concentric contraction
(c) Isotonic eccentric contraction
Fatigue
11-7
(c)
(d)
Isometric and Isotonic Phases of Contraction
Muscle Metabolism
• all muscle contraction depends on ATP
Length or Tension
• ATP supply depends on availability of:
Muscle
tension
– oxygen
– organic energy sources such as glucose and fatty acids
Muscle
length
• two main pathways of ATP synthesis
Isometric
phase
aerobic respiration
• produces far more ATP
• less toxic end products (CO2 and water)
• requires a continual supply of oxygen
– anaerobic fermentation
• produce ATP in the absence of O2
• yields little ATP and lactic acid, a major factor in muscle
fatigue
Isotonic
phase
Time
• at the beginning of contraction – isometric phase
– muscle tension rises but muscle does not shorten
• when tension overcomes resistance of the load
– tension levels off
• muscle begins to shorten and move the load – isotonic phase
Overview of ATP Production
Glycolysis
Stages of
glucose
oxidation
11-9
11-10
Modes of ATP Synthesis During Exercise
Glucose
2 ADP + 2 Pi
2 ATP
Pyruvic acid
Anaerobic
fermentation
No oxygen
available
Lactic acid
Aerobic
respiration
Oxygen
available
Glycogen:
- Stored in Liver,
Skeletal muscles,
heart
- Can be changed
into glucose
CO2 + H2O
Mitochondrion
36 ADP + 36 Pi
0
10 seconds
40 seconds
Duration of exercise
Repayment of
oxygen debt
Mode of ATP synthesis
At rest most ATP
generated by
aerobic respir. of
fatty acids -
During short intense
exercise Aerobic
respiration
using oxygen from
Myoglobin - ATP is
used up quickly until
respiratory system
catches up – until then
Phosphcreatine!!!!! –
Until it gets used up!!!!
Now muscles
using
Glycogen–
lactic acid
system
(anaerobic
fermentation)
Aerobic
respiration
supported by
cardiopulmonary
Bringing O2 to muscles
36 ATP
• ATP consumed within 60 seconds of formation
2
Immediate Energy Needs
Fatigue
ADP
ADP
• Causes of muscle fatigue
– ATP synthesis declines as glycogen is consumed
– ATP shortage slows down the Na+ - K+ pumps
– lactic acid lowers pH of sarcoplasm
• inhibits enzymes involved in contraction,
• Inhibits enzymes used in ATP synthesis
– release of K+ with each action potential causes the accumulation
of extracellular K+
• hyperpolarizes the cell and makes the muscle fiber less
excitable
– motor nerve fibers use up their Achetelcholine
– CNS fatigues ??????? less signal output to skeletal muscles
Pi
Myokinase
ATP
AMP
ADP
Creatine
phosphate
Pi
Figure 11.19
Creatine
Creatine
kinase
ATP
11-13
11-14
Oxygen Debt
Endurance
• endurance – ability to maintain high-intensity exercise for
> 4 to 5 minutes
– determined in large part by one’s maximum oxygen uptake
(VO2max)
– maximum oxygen uptake – point at which rate of oxygen
consumption reaches a plateau and does not increase further with
added workload
• When exercise stops, rate of oxygen uptake does not
immediately return to pre-exercise levels
• Because oxygen debt accumulated during exercise:
– When oxygen is withdrawn from hemoglobin and
myoglobin
– And because of O2 needed for metabolism of lactic acid
produced by anaerobic respiration
- Turn it back to pyruvic acid then to glycogen
11-15
12-49
Physiological Classes of Muscle Fibers
FG and SO Muscle Fibers
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
• slow oxidative (SO), slow-twitch, red, or type I fibers
– Lots of mitochondria, myoglobin and capillaries - deep red color
– adapted for aerobic respiration - - slow to fatigue
– long lasting twitch (~100 msec)
•
fast glycolytic (FG), fast-twitch, white, or type II fibers
– few mitochondria, myoglobin, and blood capillaries - pale
appearance
– adapted for quick responses - - fast to fatigue (lactic acid)
– rich in enzymes of phosphagen and glycogen-lactic acid systems
generate lactic acid causing fatigue
– fast twitches (~ 7.5 msec)
FG
SO
• ratio of different fiber types have genetic predisposition
11-17
Figure 11.20
11-18
3
Strength and Conditioning
Strength and Conditioning
• muscles can generate more tension than bones and tendons can
withstand
• resistance training (weight lifting)
–
–
–
–
• muscular strength depends on:
– primarily on muscle size
– fascicle arrangement
contraction of a muscles against a load that resist movement
Couple times a week
Hypertrophy
more myofilaments and myofibrils synthesized
• pennate are stronger than parallel, and parallel stronger than circular
• endurance training (aerobic exercise)
– size of motor units
– multiple motor unit summation – recruitment of many motor units
– temporal summation
• nerve impulses usually arrive at a muscle in a series of closely spaced action
potentials
– length – tension relationship (resting muscle at optimal length)
– improves fatigue resistant muscles
– slow twitch fibers produce more mitochondria, glycogen, and
acquire a greater density of blood capillaries
– improves skeletal strength
– increases red blood cell count and oxygen transport capacity of
the blood
– enhances cardiovascular, respiratory, and nervous systems
– fatigue
11-19
11-20
Cardiac Muscle
Cardiac Muscle
• characteristics of cardiac muscle cells:
• Location?
• Function?
– striated but myocytes (cardiocytes) shorter and thicker than
skelatal
• Required properties of cardiac muscle
– intercalated discs
• electrical gap junctions allow each cell to directly stimulate its
neighbors
• mechanical junctions that keep the cells from pulling apart
– Autorhythmic cells
– contraction with regular rhythm
Ca+ induced Ca+ release from Sarcoplasmic reticulum
– muscle cells of each chamber must contract in unison
Damaged cardiac muscle cells repair by fibrosis
– contractions must last long enough to expel blood
• a little mitosis observed following heart attacks
• not in significant amounts to regenerate functional muscle
– must be highly resistant to fatigue
11-21
Cardiac Muscle
11-22
Smooth Muscle
• myocytes have a fusiform shape &
one nucleus
• no visible striations
• Actin and myosin do not overlap
• z discs absent
• No troponin
• Cytoplasm has:
– autorhythmic cells!!!!
– autonomic nervous system can influence it
– slow twitches!!
• maintains tension for about 200 to 250 msec
– uses aerobic respiration almost exclusively
• rich in myoglobin and glycogen
• has especially large mitochondria
- dense bodies = Like z disks
– protein plaques on the inner face of
the plasma
– cytoskeleton of intermediate
filaments: attach to the membrane
plaques and dense bodies
– highly fatigue resistant
11-23
• Ca2+ needed for muscle contraction
comes from ECF
• ANS controlled
• capable of mitosis (hyperplasia)
Plaque
Intermediate filaments
of cytoskeleton
Actin filaments
Dense body
Myosin
11-24
4
2 Types of Smooth Muscle
2 Types of Smooth Muscle
• single-unit smooth muscle
(visceral muscle)
Autonomic
nerve fibers
• multiunit smooth muscle
Autonomic
nerve fibers
– myocytes are electrically
coupled to each other by gap
junctions
– occurs in some of the largest
arteries and pulmonary air
passages, in piloerector
muscles of hair follicle, and
in the iris of the eye
– stimulate each other and a large
number of cells contract as a
single unit
– i.e., each cell does NOT have
to be stimulated directly by a
motor neuron
Synapses
– Single neuron stimulates
many myocytes
• i.e., like a motor unit
Varicosities
Gap junctions
(a) Multiunit
smooth muscle
Figure 11.21a
(b) Single-unit
smooth muscle
11-25
Figure 11.21b
11-26
Stimulation of Smooth Muscle
Layers of Visceral Smooth Muscle
• Involuntary and can contract without nervous stimulation!!
– chemical stimuli: hormones, carbon dioxide, low pH, and oxygen deficiency
– in response to stretch
– Cold stimulus – (aeola and scrotum)
– Stretching - peristalsis
• most innervated by ANS nerve fibers
– can trigger and modify contractions
– Neurotransmitters: acetylcholine or norepinephrine
Mucosa:
Epithelium
Lamina propria
Muscularis
mucosae
Muscularis externa:
Circular layer
Longitudinal
layer
Figure 11.22
11-27
Stimulation of Smooth Muscle
11-28
Contraction and Relaxation
• contraction is triggered by Ca+2, ATP, and sliding thin
past thick myofilaments
Autonomic
nerve fiber
• contraction begins in response to Ca+2 that enters the
cell from ECF
– voltage, ligand, and mechanically-gated (stretching)
Ca+2 channels open (Ca+2 enters cell)
Varicosities
• Ca+ binds to calmodulin
– activates myosin light-chain kinase – adds phosphate to
myosin head
– This activates myosin ATPase - hydrolyzing ATP – allowing
myosin head to move Actin
– thick filaments pull on thin ones, this pulls on dense bodies and
membrane plaques
Mitochondrion
Synaptic
vesicle
Single-unit
smooth muscle
Figure 11.23
11-29
11-30
5
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enzyme
Component
of myosin
12-81
Stretching Smooth Muscle
Plaque
• stretch can open mechanically-gated calcium channels in the
sarcolemma causing contraction
– peristalsis – waves of contraction brought about by food distending
the esophagus or feces distending the colon
• propels contents along the organ
Intermediate filaments
of cytoskeleton
• stress-relaxation response (receptive relaxation) -helps hollow organs
gradually fill (urinary bladder)
– when stretched, tissue briefly contracts then relaxes – helps prevent
emptying while filling
Actin filaments
Dense body
Myosin
(b) Contracted smooth
muscle cells
Figure 11.24
(a) Relaxed smooth muscle cells
11-33
11-34
Contraction and Stretching
• smooth muscle contracts forcefully even when greatly stretched
– Remember what happens when Skeletal muscle is stretched?
• plasticity – the ability to adjust its tension to the degree of stretch
– a hollow organ such as the bladder can be greatly stretched yet not become
flabby when it is empty
11-35
6